A load limiting device for a seat belt retractor, for example, a speed-controlled load limiting device, is known from WO 2006/108451 A1 filed by this applicant; said device features a plurality of pendulum masses disposed on the frame of the seat belt retractor. The masses are seated so as to rotate on the retraction frame and during the load-limited unwinding of the seat belt, engage alternately with two teeth into a toothed ring connected to the belt shaft. Due to the pendulum masses disposed on the retractor frame, the seat belt retractor as a whole is complicated and expensive to manufacture. Furthermore, due to the pendulum masses the seat belt retractor requires a comparatively large installation space.
In the German patent application 10 2008 049 931.5 also filed by this applicant, an improved load limiting device operating on the basis of the same physical principle is described, in which the two toothed parts are moved with respect to each other in a force-limited manner and one of the parts executes a “wave shaped” forward motion in which the toothed parts alternately are engaged and disengaged. The pendulum masses are replaced by the part executing the wave-shaped forward motion, so that the load limiting device described therein requires a significantly smaller installation space and displays a significantly simpler structure. In one exemplary embodiment the part executing the wave-shaped forward motion is realized by a toothed disc which is friction-locked to the belt shaft.
Furthermore, from DE 10 2009 010 435 A1 a seat belt retractor with a speed-controlled load limiting device is known, in which for the activation thereof, the load limiting device is secured to a part on the frame of the seat belt retractor and is connectable to another part on the belt shaft. The part by which the load limiting device is connectable to the belt shaft is designed as a rotatable toothed ring in which a locking pawl seated on the belt shaft engages. During the load-limited belt extension motion, the torque from the toothed ring is directed via axial fingers into a toothed flywheel which then begins a rotational motion with an overlapping, transverse-directed oscillatory motion which alternately engages and disengages with teeth arranged on the frame of the seat belt retractor. During this rotational motion the oscillating disc is periodically accelerated and decelerated, so that the destruction of energy underlying the load limiting principle is effected. One disadvantage of this design is seen in that, upon activation of the blocking device, the toothed ring must have a particular alignment to the engaging locking pawl so that the locking pawl does not undergo false locking against the peak of the tooth. Therefore, the engaging motion of the locking pawl must be synchronized with the alignment of the toothed ring, which in turn can be problematic since the alignment of the toothed ring itself is established by its assembly to the retractor frame. Thus, the alignment of the toothed ring has a certain, unavoidable tolerance due to manufacturing tolerances for the toothed ring itself and its attachment points to the retractor frame and these tolerances complicate the synchronization of the motion of the locking pawl.